Searching for charged lepton flavour violation with the Mu3e experiment -- Pixel Sensor

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The main goal of the Mu3e Phase I experiment is the search for the charged lepton flavor violating decay gamma -> e e e with a sensitivity of BR(gamma -> e e e)< 5 × 10−15 at 90% confidence level (CL), corresponding to a factor 200 improvement compared to the existing bound. High precision tracking is required to suppress the large background for muon stopping rates up to 10^8 muons per second. The position and momentum of electrons will be measured by means of a four layer pixel tracking detector (PTD) which exploits High-Voltage CMOS Monolithic Active Sensors (HV-MAPS). The high rate of background determines the requirements on the tracking detector system and the pixel sensor. The pixel sensor has to provide high position resolution, good time resolution, low power consumption and high rate capability. In addition, the pixel sensor should be as thin as possible since the thickness determines the amount of multiple Coulomb scattering in the detector and also the rate of Bhabha scattering.For the Mu3e experiment, a dedicated HV-MAPS sensor, the MuPix has been developed over many years. MuPix is a fully monolithic sensor which integrates on-chip digital readout. The MuPix sensor is close to production readiness and the production, qualification, commissioning and calibration of final sensor is main subject of the PIX project.